On the Repeatability of HVSR Determinations from Ambient Seismic Noise Measurements

The horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise is a popular tool for determining near-surface seismic structure for geotechnical applications. The most important information from an HVSR is the frequency of the peak of the fundamental soil resonance, often called the peak frequency. The peak frequency can be used to infer the average surficial seismic S-wave velocity if the depth to the reflecting surface is known, or the depth to the reflecting surface can be determined if the average S-wave velocity of the layers above the surface is known. In this study, we made multiple measurements of seismic noise at the same sites on an earth embankment dam near the Boston College campus to document the variability of the HVSRs with time, weather conditions, and waver levels behind the dam. The spectra are quite similar among all of the measurements made at a site, although there are variations if different peaks in the spectra from one measurement to the next. The values of the spectral peaks vary by only a few percent among all of the measurements at a site, with the exception of one site where the fundamental frequency has two nearly equal spectral peaks. The time of year and the weather do not seem to affect the HVSRs at a site, although the spectral shapes do show some dependence on level of the water behind the dam. The peak frequency and the major features of the HVSR at a site appear to be well determined independent of when the ambient noise data are acquired.